Add GitHub actions

[suve]

This patch adds a single GitHub Actions workflow with three jobs:

• first one runs on Ubuntu 20.04
• second one runs on MacOS 11
• third one runs on Windows Server 2022

Each of these installs FPC (using apt for Ubuntu, brew for MacOS and choco for Windows) and compiles all the SDL2 units.

[Free-Pascal-meets-SDL-Website]

Let's try this :-)